The claim, posted on a Web site sympathetic to the militants, could buttress the suspicions of Russian investigators who have been tracing the attack to Islamist separatists in Russia's volatile North Caucasus region.

The separatist statement, issued on behalf of Chechen leader Doku Umarov, said the bombing on Friday of a Moscow-St. Petersburg express train was carried out on his orders.

"We declare that this operation was prepared and carried out … pursuant to the order of the emir of Caucasus Emirate," or Umarov, it said.

Umarov is thought to head a network of separatist cells across the mainly Muslim North Caucasus region that are fighting to break free from Moscow's rule. The rebels are blamed for regular attacks on law enforcement officials since the end of two wars in Chechnya.